A corridor-based flight mode transition strategy for agile ducted-fan tail-sitter UAV: Altitude-hold transition

As an attractive transition approach, the altitude-hold transition is a special type of super-maneuvering and the vertical/horizontal flight mode transition that an agile aircraft conducts at fixed altitude. However, it is still challenging to implement an autonomous control of the altitude-hold transition while the existing optimal transition planning methods cannot avoid an evident altitude change during the transition process. This paper proposes a corridor-based flight mode transition strategy and presents a successful flight demonstration of the altitude-hold transition on a small ducted-fan tail-sitter unmanned aerial vehicle. In the proposed corridor-based methodology, we model and analyze the transition corridor, concentrate on the dynamic characteristics of the altitude-hold transition, and emphasize that a valid transition trajectory should be governed by its transition corridor. The identified transition corridor reveals that for a given velocity trajectory, the solution for the corresponding trajectories of pitch angle and thrust is unique. Based on this, the transition trajectory generation problem is addressed simply on the velocity-acceleration plane. Furthermore, a proper flight control scheme is devised to track the generated transition trajectories. Finally, the effectiveness of the proposed method is verified through practical flight tests, in which the altitude change is less than 1.1 m during the entire transition course.